Add pristine-ubuntu-action to hosted runners

[Copilot]

Prevent GitHub-hosted runner updates from breaking workflows by cleaning up pre-installed bloat (15-20 GiB freed).

Changes

• Added lengau/pristine-ubuntu-action@v0 as first step in setup-python-runner/action.yaml
• Conditionally runs only on GitHub-hosted Linux runners: runner.os == 'Linux' && runner.environment == 'github-hosted'
• Self-hosted runners automatically skip this step

Affected Workflows

Applies to all workflows using setup-python-runner:

• test-python.yaml (reusable workflow)
• self-test-setup.yaml
• Any downstream callers

steps:
  - name: Pristine Ubuntu
    if: ${{ runner.os == 'Linux' && runner.environment == 'github-hosted' }}
    uses: lengau/pristine-ubuntu-action@v0
  - name: Start apt update
    # ... rest of setup

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• https://api.github.com/repos/lengau/pristine-ubuntu-action
  • Triggering command: /usr/bin/curl curl -s REDACTED (http block)
• https://api.github.com/repos/lengau/pristine-ubuntu-action/tags
  • Triggering command: /usr/bin/curl curl -s REDACTED (http block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Use the pristine-ubuntu action on hosted runners</issue_title>
<issue_description>### What needs to get done

On hosted runners we should use a version of https://github.com/lengau/pristine-ubuntu-action/.

Why it needs to get done

Prevent updated runners from breaking us yet again. </issue_description>

<agent_instructions>That action should be the first thing that's run on the github-hosted runners. It should not run on self-hosted runners at all.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Use the pristine-ubuntu action on hosted runners #100

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